Patent GB-1230927-A (GB)
IPC: H04Q11/04
Filed: 1969-06-23
Abstract: 1,230,927. Automatic exchange systems. Y. LE GOFFIC, M. LE DORH and F. ARNOUAT. 23 June, 1969 [21 June, 1968], No. 31690/69. Heading H4K. Substations have four-wire circuits to a timedivision-multiplex exchange and transmit and receive delta-modulated signals, the deltamodulated speech sent from the exchange to a substation being used to synchronize a clock controlling a delta-modulator transmitting to the exchange where the received pulses of the delta-modulation train are regenerated and stretched to have a duration equal to that of an exchange multiplex cycle. The modulation at a substation is such that in each period of the controlling clock a comparator determines whether the output of an integrator is less than or greater than the speech signal, the integrator output being increased or decreased by one step according to the comparator result. For increases in integrator output a pulse is produced for transmission to the exchange. At the exchange the substation lines are terminated on line circuits which regenerate the delta-modulation pulse train reducing the length of each pulse to a duration of 1 Ásec. The exchange clock, which determines the multiplex cycle frames, runs slightly faster than the clocks at the substations when these are allowed to run free of synchronization. In circumstances in which speech transmitted to a substation originates from a saturated modulator, it is possible that about 30 exchange cycles may pass by without the substation receiving an actual pulse in the delta-modulated pulse train and during these 30 cycles the modulator at the substation runs at its slower rate, unsynchronized. The object of the exchange line circuits is to produce ON signals for the duration of the next following exchange multiplex cycle in response to each received delta-modulation pulse and to present OFF signals in those exchange cycles preceded by no actual deltamodulation pulse. When, however, a received delta-modulation pulse comes from an unsynchronized modulator and advances in phase relative to the exchange cycles to occur after the onset of the multiplex cycle in which its resultant ON condition should be effective, the line circuit switches over to a mode of operation in which the ON or OFF condition relayed in response to the preceding deltamodulation signal is left in force for a further period of a half-multiplex cycle, despite the consequent error in scanning the condition at the exchange, while the pulse giving rise to the problem and subsequent signals are converted to ON, OFF, conditions presented during periods of a multiplex cycle shifted forward by a halfcycle. If received delta-modulation pulses advance even further in phase to occur after the onset of the shifted period of a multiplex cycle the line circuit introduces a further shift of a half-cycle and so on. Delta-modulation pulses switched over the exchange in time slots associated with the substations are fed to the line circuit of the receiving substation and are retimed to occur as pulses in the first or second half of the exchange multiplex cycles according to the position of the time slot employed in the exchange. Where the multiplex cycle has a period of 17À85 Ásecs. gaps in synchronization at the substations are said to produce no interference with the transmission of delta-modulation pulses if they do not produce displacements between substation and exchange clocks in excess of 5 Ásecs. From this condition it is possible to calculate permissible substation clock frequencies.